Add the ability to (properly) handle subfolders

Right now subfolders are only partially supported, in the sense that their contents are displayed under the same category as the top folder. So although the subfolders themselves aren't shown, at least their apps get displayed rather than hidden.

This, of course, is not an ideal solution. But it suits the majority of users, who do not organize their menu into subfolders.

Until I can figure out a way to properly support subfolders while still keeping the UI simple, this is how it's going to stay. If you have any suggestions how this could be done, please post them in the whiteboard below.